THE ACRANIATES. 



397 



way directly united with the vertebrate stock, and in no sense to be regarded as 

 the ancestors of the vertebrates. 



From every point of view, and from every point of departure, the evidence 

 leads more and more decisively to this conclusion. The vertebrates, however 



.. -A -.Notochoi-d. 



_ Gill Clefts 

 naemostoirLa 



.Tarasittc. 



. Endo crarxtvuna,. 



.Fore Tar am abs 



or cCea'. 



._ Telocoel 



Free . 

 "Fixed. 

 _CepHal stalk.; 



oct. 



FIG. 266. Diagram to illustrate the probable interrelations of the acraniata, and their origin from a small 

 nauplius-like arthropod ancestor. The characteristic shape of each division and its normal orientation to its 

 point of attachment is indicated. The neural surface may be identified by the brain and nerve cord, in black. The 

 principal characters of each division are indicated by the presence of a + or o sign opposite the descriptive term 

 on the right. When a character may be either present or absent, or doubtful, the plus sign is enclosed in a circle. 

 The more specialized characters stand, as far as practical in such a diagram, at the top of the list. 



far back we may go, do not lead to the acraniates, and the acraniates do not 

 take us back to the same stock as that from which the vertebrates arose. Indeed 

 all the acraniates are chiefly notable for the absence of those very structures 

 and modes of growth so characteristic of the craniates. In the acraniates a differ- 

 ent set of organs are emphasized, and the general direction of evolution is different. 



